Boston's Dragnet and the Art of the Manhunt

House-to-house searches, heavily armed police, a neighborhood under lockdown, and helicopters overhead. Here's the insider's guide to how police conducted the most intensive containment operation in U.S. history.

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America was transfixed by the hunt for Dzhokar Tsarnaev, one of two accused Boston Marathon bombers who killed three people and wounded more than 170. The images of heavily armed personnel sweeping the town of Watertown, Massachusetts, captured one of the largest, most complex operations of its kind in U.S. history.

"There's a lot about that search and containment that was the same for all sorts of different bad guys, guys who rob banks and steal cars," he says. "But these guys are terrorists. They are throwing IEDs (improvised explosive devices) at officers. I've never heard of that. . . . This was not the normal police foot chase and search."

When Dzhokar was on the loose, the police set in motion a containment operation that begins as soon as police lose sight of a suspect. To police, containment means having a line of sight on all of a suspect's exits. Two observant cops can cover an entire block if positioned on diagonal corners. Vehicle checkpoints, K-9 searches, and—in a case as dramatic as Boston's—a shutdown of public transit can further seal off an area.

The size and urgency of the containment depends on the crime and the level of danger the target represents. Dzhokar's brother, Tamerlan, died while charging police, armed with explosives, so the reaction was sweeping. "Desperation goes hand in hand with the size of the perimeter," Schonely says. "And in that way, these two brothers were off the chart."

Officers on the scene typically determine the size of a perimeter. "It's that lone police officer who was on that street, who was just involved in a gun battle," Schonely says. "The decisions are made by street police officers, deputies, agents, and whoever happens to be there, on what to do and how to do it." The immediate goal of a containment operation is to make the suspect stop moving. "If you don't find a way to contain them, most of these guys will keep moving; they'll get on a bus or a train, they'll call a friend," Schonely says. "If he keeps moving, his chances of getting away increase."

The situation in Boston was extreme. Police had to assume the remaining suspect was armed with guns and explosives. Even more troubling, he had fled into a residential area. The police could call on a huge number of officers to seal off the neighborhood, and so they did. The size of the containment area measured about 20 blocks. Thosuands of officers swept the area. Yet this is more an art than a science—even with all these resources, Dzhokar was located one block outside the designated containment area. Luckily for police, he had stopped running and sought refuge in a boat.

Once the containment perimeter is established, the search begins. In Boston, officers in tactical gear went door to door in search of Dzhokar. In a manhunt, the police can access lawns and peer in windows, but the legal protection of probable cause still exists. If there is no sign of trouble—a blood smear, sign of forced entry, cries for help—the police cannot enter a home no matter who's on the loose. "We still have to follow the law," Schonely says. "Without probable cause, the police can only ask residents to come out for permission to enter."

"The officers were asking, 'How should we deal with the residents?' and we made it clear: 'As courteous and as professional as possible, yet getting the job done,'" Boston Police Department Superintendent William Evans says.

Searching a populated area is certainly a challenge, but it can also work to the police force's advantage. "It can be a plus or a minus," Schonely says. "All those people are put in danger, but they all have eyes and ears." Indeed, Dzhokar was snared by the report of a citizen who saw signs of someone on his property—the covering over his boat was out of place.

One benefit of containment is that it provides time to marshal extra forces for the search. A systematic plan is ideal, with patrol officers backed by police dogs and helicopters. In Boston, a Massachusetts state police helicopter used a Forward Looking Infrared (FLIR) sensor to spot heat signatures.

Schonely, who works in police helicopters over Los Angeles, says the best use of FLIR is in advance of the foot patrols, where it can give the officers below information on the area they are entering. But a heat signature is rarely clear enough to identify as a human being, he says, making the clear image of Dzhokar in the boat a lucky break. Most of the time FLIR shows only a glimmer of heat that can be a reflection, an animal, or even household equipment. "Most officers on the ground think air units can do more than they can. That can make them overconfident," Schonely says.

When the suspect is found, police are supposed to pause, assess the situation, and formulate the endgame. With the manhunt over, it's time for the takedown.